The operation of axial flow gas turbine engines involves the delivery of compressed air from the compressor section of the engine to the combustion section of the engine, where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. The turbine is contained and circumscribed by a shroud, such that the shroud is adjacent to the tips of the turbine blades. The shroud serves to channel the combustion mixture through the turbine so as to ensure that the bulk of the mixture entering the turbine drives the turbine. However, a small portion of the air is able to bypass the turbine through a radial gap present between the turbine blade tips and the shroud. Accordingly, the efficiency of a gas turbine engine is dependent in part on the ability to minimize leakage of compressed air between the turbine blades and the shroud of the engine's turbine section.
To minimize the gap between the turbine blade tips and the shroud, shrouds often undergo a final rotor grind such that the turbine rotor assembly closely matches its shroud diameter. However, manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Furthermore, during the normal operation of an aircraft gas turbine engine, the turbine blades may rub the shroud as a result of a hard landing or a hard maneuver of the aircraft. Any rubbing contact between the turbine blade tips and the shroud will abrade the tips, tending to further increase the gap between the shroud and turbine blade tips, thereby reducing engine efficiency. Accordingly, it is well known in the art to cover the surface of the shroud adjacent the blade tips with an abradable coating, such that the coating will sacrificially abrade away when rubbed by the turbine blades. Inherently, as the coating is removed, the gap between the blade tips and the surface of the shroud will increase, necessitating restoration of the coating in order to maintain desirable aerodynamic efficiencies associated with a smooth rub surface and a small gap between the rub surface and the turbine blades.
Various processes have been employed to restore shroud rub surface coatings. In the high pressure section of a turbine, the shroud is often a cobalt-base superalloy. Particularly suitable coatings for the abradable rub surface of a cobalt-base superalloy shroud include environmentally-resistant compositions such as MCrAlY, where M is cobalt, nickel and combinations thereof as a result of the cobalt-base superalloy substrate. A typical method of removing MCrAlY coatings from a shroud is by abrasion, e.g., grit blasting, or treatment with an acidic solution. An example of an acidic solution used in the prior art contains, by volume, about 28% ferric chloride (FeCl.sub.3), about 10% phosphoric acid (H.sub.3 PO.sub.4) and about 10% nitric acid (HNO.sub.3), the balance water. However, this solution is very aggressive to a cobalt substrate if the treatment is not performed under strict guidelines (e.g., temperature and composition), and when properly used requires up to about twelve hours to strip an MCrAlY coating. Furthermore, the solution must be replaced at relatively short intervals to maintain its effectiveness.
Another notable shortcoming is that this solution is not effect for removing nickel-base alloys of the type often present on the shroud and other cobalt-base superalloy components of a gas turbine engine, such as nickel-base braze alloys that serve to attach cover plates, inserts, etc., to such components. As a result, removal of the braze alloy to permit refurbishment of a shroud or other cobalt-base superalloy component requires a second treatment in hot nitric acid for an extended duration, followed by a third treatment with a hot potassium permanganate (KMnO.sub.4) solution. Such treatments may continue for up to forty hours or more until the cobalt-base substrate is ready for further processing, e.g., application of a new abradable coating.
Similar difficulties are encountered when processing other cobalt-base components of gas turbine engines. For example, cobalt superalloy vanes are typically protected by an environmental coating or bond coat such as a diffusion aluminide or MCrAlY. To repair the vane, such coatings must be removed, as must various inserts attached to the surface of the vane, typically with a nickel-base braze alloy. Nitric acid solutions have been successful in softening nickel-base braze alloys during the processing of cobalt superalloy vanes, but final processing requires mechanical removal of the braze alloy and insert, such as by grit blasting. However, grit blasting is very labor intensive and detrimental to the component substrate because some of the substrate is inevitably removed by the grit blasting process. With repetitive use, components stripped in the manner described above are no longer usable.
From the above, it can be appreciated that it would be desirable if a process were available for rapidly stripping a cobalt-base substrate with reduced risk to the substrate. It would be particularly desirable if such a process were capable of removing various metallic compositions, particularly aluminum-containing coatings and nickel-base compositions, from the surfaces of gas turbine engine components.